1. Field of the Invention
The present invention relates to a liquid crystal device and a method for driving the same, more particularly to a liquid crystal device having a high response speed and a method for driving the same.
2. Description of the Related Art
A conventional projection-type liquid crystal display device using a liquid crystal device is capable of obtaining a picture of a large size with relative ease by irradiating light onto a liquid crystal display so as to project the light onto a screen. There are two methods for obtaining a color display: a method in which a projected light beam is split into red, green and blue light beams, and a liquid crystal display device is used for each of the colors (simultaneous additive color mixing); and a method in which red, green and blue pixels are provided in a liquid crystal display device as in a direct-view type (juxtapositonal additive color mixing). However, both methods have problems. With the former method, while high resolution can be obtained with ease, it is expensive to realize such a liquid crystal display device. As shown in FIG. 29, light beams radiated from a lamp 1 as a light source propagate through three optical paths, that is, a dichroic mirror for a red light beam 2, a dichroic mirror for a green light beam 3, and a dichroic mirror for a blue light beam 4. The light beams pass through liquid crystal panels 5, 6 and 7, respectively, and are output from a lens 8. As described above, since the three liquid crystal display panels 5, 6 and 7 are used, the optical system for projection becomes complex and large in size as a whole system. Moveover, if a defective pixel exists even in one liquid crystal display panel among the three, a bright spot with a single color or a mixed color occurs in the projected image at a portion corresponding to the defective pixel. On the other hand, while the latter method is inexpensive, it has a problem in that the quality of displayed image is deteriorated unless the size of red, green and blue pixels in a projected image is smaller than the spatial resolution of human eyes. As one of the methods for solving the above problem, a field sequential color mixing method, a color mixing method by using a field sequential addressing method, is known which can display red, green and blue with one pixel. The characteristics of high precision and high brightness of the field sequential color mixing method have the following features.
(1) The principle of displaying color images by the field sequential color mixing method is the same as that by the simultaneous additive color mixing method. Therefore, the field sequential color mixing method provides high precision images.
(2) In the case where the liquid crystal panel has a defective pixel, the defective pixel is displayed as a white or black point. The white or black point is less conspicuous than the colored bright point. Accordingly, even if the defective pixel exists in the liquid crystal panel, the quality of the displayed image is not deteriorated.
(3) Full-color display or multi-color display can be realized with a single liquid crystal panel, and therefore the optical system can be miniaturized and lightened. Since it is not necessary to use a plurality of light shutters as in the simultaneous additive color mixing method, it is possible to miniaturize the system and lower the fabrication cost.
As described above, a compact and light color liquid crystal display device with high brightness and high precision, which is excellent in display quality, can be obtained with the use of the field sequential addressing method.
In the case of the field sequential addressing method, however, the time allowed for displaying images corresponding to each of Red, Green and Blue in one field is in the range of 5 to 6 msec. At the present time, the response time of Twisted Nematic (TN) mode used in an active matrix liquid crystal display device is approximately several tens msec. In the case of FIG. 30, the response times for rise and decay are 39.1 msec and 35.1 msec, respectively. Considering that the response time in a liquid crystal display mode, which utilizes optical switching between on/off states in the vicinity of a threshold voltage, is the same as or longer than that for the TN mode, it is practically impossible to realize the color liquid crystal display device of the field sequential addressing method.
As a conventional liquid crystal display mode having high-speed response, Surface Stabilized Ferroelectric Liquid Crystal (SSF-LC) mode is well-known (N. A. Clark and S. T. Lagerwall; Appl. Phys. Lett., 36,899: 1980). The feature of SSF-LC mode is as follows: the ferroelectric liquid crystal molecules have spontaneous polarization, and the display is performed by utilizing the property of the liquid crystal molecules which change their orientations so that the polarity of the spontaneous polarization and the polarity of an applied electric field are parallel with each other.
Regarding a liquid crystal display method with high-speed response other than the ferroelectric liquid crystal mode, Japanese Patent Publication No. 56-51352 describes that the response speed is increased by applying a voltage close to the threshold value and a voltage close to the saturation voltage at which an optical characteristic of the liquid crystal is saturated.
Another high-speed response display mode using nematic liquid crystal is described in a publication (Nematic liquid crystal modulator with response time less than 100 .mu.s at room temperature: Shin-Tson Wu; Appl. Phys. Lett. 57(10), 986 1990). The method for driving the liquid crystal display described in the publication is shown in FIG. 31. A voltage (V.sub.off) is continuously applied to the liquid crystal molecules such that the orientational deformation of the liquid crystal molecules from the initial orientation state becomes the largest. In this state, the transmittance of the liquid crystal display is zero. Then, zero voltage (V.sub.0) is applied to the liquid crystal molecules such that the orientational deformation of the liquid crystal molecules is relaxed. The transmittance is changed by varying a time period for applying zero voltage, thereby obtaining a gray-scale display. The relaxation process of the liquid crystal molecules which are orientationally deformed is often compared to the movement of a spring. The potential energy due to the interaction of the liquid crystal molecules becomes higher as the degree of the orientational deformation of the liquid crystal molecules becomes larger. As a result, the liquid crystal molecules in the highly deformed orientation state relax with extremely high speed.
However, the conventional liquid crystal display mode using the ferroelectric liquid crystals (FLC mode), such as SSF-LC mode, suffers from the following problems. In addition to the difficulty in controlling the orientation of the ferroelectric liquid crystal molecules, the orientation of the molecules is easily destroyed by a mechanical shock. Moreover, since the orientation of the ferroelectric liquid crystals is in a bistable state, it is difficult to obtain the gray-scale display.
As for the driving method described in Japanese Patent Publication No. 56-51352, it is not capable of displaying gray-scale. Moreover, since the degree of the change in the orientation state of the liquid crystal molecules is large, it is difficult to increase the response speed higher than that for the TN mode.
The method in which the relaxation of the orientational deformation of liquid crystal molecules is adjusted by varying the voltage unapplied period in order to obtain the gray-scale display, such as the above-mentioned high-speed response display mode using the nematic liquid crystal, cannot be adopted to matrix driving used for commercial liquid crystal display devices and the like.